Rise and fall of a hydrothermal system: A tale of metalliferous sediments (TAG hydrothermal field, MAR, 26°N)

Abstract

Hydrothermal sediments, or metalliferous sediments, are linked to hydrothermal activity and are found in the vicinity of seafloor hydrothermal systems. Their study allows for the geochronological reconstruction of hydrothermal systems and further understanding of the geochemical alteration processes of seafloor massive sulphides (SMS). As they are rich in copper, zinc and gold, hydrothermal sediments are also potential mineral resources and excellent indicators for SMS ore deposits in deep-sea mineral exploration. Furthermore, formation of these sediments and the subsequent diagenesis, have an impact on global elemental cycles and budgets as they retain or release metals. This study investigates the origin, nature and diagenesis of metalliferous sediments present at the TAG hydrothermal field (TAGHF, 26°N, Mid-Atlantic Ridge) in order to address the geochronological evolution of the hydrothermal system. For this, metalliferous sediments have been collected using gravity coring, rock drilling and mega-coring in distinct seafloor environments at the TAGHF: from the top of inactive hydrothermal mounds, at the base of hydrothermal mounds and in several hundred meters-distant depositional basins and channels. Dating by radiocarbon isotopes, oxygen isotopes and palaeomagnetism shows that high temperature hydrothermal activity from a number of individual vent sites across the TAGHF was synchronously suppressed. This suppression coincides with the last glacial termination, ca. 12,500 years ago, when the sea-level rose. In addition to the synchronous cessation, at least three vent sites show signs of synchronous rejuvenation in the last 10,000years. These synchronous suppression and reactivation support the hypothesis that a unique heat source below the ridge feeds several vents simultaneously. After the cessation of high-temperature hydrothermal activity, sediments underwent diagenesis and are influenced by low-temperature hydrothermal fluids. The mineralogical and geochemical analyses of these sediments and their pore waters demonstrate that post-deposition processes vary according to their environments of deposition. In distal depositional basins, metals are rapidly buried in thick turbiditic deposits up to several metres thick. Copper, cobalt and manganese are mobilized by diagenetic redox processes, and re-distributed to be eventually preserved in sulphides, or oxides closer to the sediment/seawater interface. These processes surpress metal release into the ocean and help preserve the metal tenor, resulting in a considerable resource potential in the sediments of this area. On the flanks and at the base of extinct hydrothermal sites (mounds), recurrent sulphidic debris-flows also hold a high metal tenor (e.g. up to 28 wt% Cu), yet exposure to seawater dissolves and mobilises these metals into pore waters, which are eventually released into the ocean. On the top of the extinct hydrothermal mounds, oxidised hydrothermal sediments, here referred to as seafloor gossans, are dominantly derived from sulphide chimney material. They have experienced complete oxidative weathering by seawater, slow burial, and are capped by pelagic carbonate sediments. Down-core changes in their lithology suggest deeper sediments, close to the SMS ore body, are dominantly composed of fine hematite and quartz, which precipitated from low to moderate temperature hydrothermal fluids that are themselves implemented in the formation of a subjacent silicified layer of jasper that preserves the underlying main ore body. This thesis explores the fate of seafloor massive sulphides from a well-known area on the Mid-Atlantic Ridge, and hence aims to increase the knowledge of metal preservation in hydrothermal sediments in general, and the controls on fluctuation of hydrothermal activity and mineralisation.

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